A low noise amplifier (LNA) is often utilized in a communication system. The main function of the low noise amplifier is to amplify weak signals received from an antenna and process the received signals to be low noise figure signals.
Recently, the researches for utilizing only one terminal to support kinds of wireless communication standards grew vigorously. The technique in this field is mainly classified into two types. One type is to connect a plurality of low noise amplifiers in parallel, and each low noise amplifier supports a specific frequency in a communication system. This type only supports one kind of wireless communication standards. The other type is a low noise amplifier capable of supporting kinds of wireless communication standards or a wide band of frequencies. However, this type of low noise amplifier has high noise figure and the gain thereof is easily decreased.
Some low noise amplifiers use a load of low impedance in order to support a wide frequency range. This type of low noise amplifier has problems of decreasing the gain and raising high noise figure. In addition, some low noise amplifiers utilize a plurality of inductors for input matching and add band pass filters in the input portion. However, the inductors sizes may be huge. Integrating the plural inductors will lead to low efficiency. Moreover, parasitic resistance resided in the chip may raise noise figure.
To make input matching to meet with operating frequencies, a circuit for input matching can be added in the input portion. However, the added circuit will increase parasitic capacitance or resistance in the input portion. This may decrease the gain.
In addition, the added circuit will generate noise signals. When the noise signals are propagated to the output portion, it may result in high noise figure and aggravate gradually.
The present invention provides a re-configurable low noise amplifier utilizing feedback capacitors. The output portion of the low noise amplifier is maintained at high impedance and feeds the output impedance, which is amplified according to a gain determined by a capacitance distributor, back to the input portion. The input portion is able to receive signals in a wide band of frequencies and the output portion is able to output signals of various frequency bands. The present invention fulfills high gain and low noise figure without adding an additional circuit in the input portion.
To support signals in a wide band of frequencies, the low noise amplifier has output transistors in the output portion and the output transistors have a switch function. To satisfy different standards, the low noise amplifier has capacitor switch cells in the output portion and the capacitor switch cells are able to slightly adjust harmonic frequencies according to a predetermined mode so as to prevent lowering the gain and increasing noise figure. In addition, the low noise amplifier is re-configurable.
In the present invention, the re-configurable low noise amplifier utilizing feedback capacitors comprises an input transistor, an output portion, and a capacitor feedback part. The input transistor has one terminal connected to a capacitor and a first inductor and is used for amplifying a received signal to be an amplified signal. The output portion is configured to output the amplified signal. The output portion has a plurality of output transistors and each output transistor has one terminal connected to another terminal of the input transistor. Each output transistor is configured to be switched according to a frequency band of the amplified signal in a wide band of frequencies. The capacitor feedback part has a capacitance distributor arranged at the gate electrode of the input transistor and connected to another terminal of each output transistor. The capacitance distributor is configured to adjust a gain and to match an input impedance and an output impedance of the re-configurable low noise amplifier with feedback.
In addition, the output portion of the present invention comprises a plurality of inductors respectively coupled to the output transistors. The output transistors are separately arranged according to a plurality of specific frequency bands. Each output transistor is controlled ON/OFF by applying a bias voltage to the gate electrode of each output transistor for switching the frequency band of the amplified signal to one specific frequency band corresponding thereto.
In addition, the output portion of the present invention further comprises a plurality of variable capacitors. The plural variable capacitors form common nodes with the inductors of the output portion. The plural variable capacitors respectively connect to another terminal of the output transistors. The plural variable capacitors are used for adjusting harmonic frequencies corresponding to the plural specific frequency bands.
In addition, the capacitor feedback part comprises a second capacitor and a plurality of first capacitors. The plural first capacitors respectively couple to common nodes formed by a plurality of inductors of the output portion and the output transistors, and respectively couple between the common nodes and the gate electrode of the input transistor. The second capacitor couples between the gate electrode of the input transistor and ground.
The present invention has advantages as follows. The output impedance feeds back to the gate electrode of the input transistor so that the output portion at high impedance maintains without adding an additional circuit in the input portion. Adding the additional circuit may affect noise figure of the main circuit. When input matching, the frequency variation of signals in the output portion can instantly reflect to the input portion. Capacitance distributors are used in the present invention. By altering the capacitance value of capacitors of the capacitance distributors, the gain is easier to be adjusted. The low noise amplifier of the present invention not only makes applicable to kinds of communication standards, but also fulfills high gain and low noise figure.
In addition, the present invention has further advantages as follows. The low noise amplifier of the present invention includes output transistors having a switch function, and capacitor switch cells for adjusting a harmonic frequency. A specific frequency band in a wide band of frequencies is selected by the switch function of the output transistors. The capacitor switch cells are able to adjust an appropriate harmonic frequency according to the frequency range devised in kinds of communication standards. Therefore, the gain of the low noise amplifier is increased.
In addition, the present invention has other advantages as follows. The low noise amplifier of the present invention is able to shrank chip area and reduce cost of manufacture. Therefore, the present invention conforms to the tendency of device miniaturization. The present invention satisfies the characteristic of operation in a wide band of frequencies. In the mean while, high gain and low noise figure are achieved. In comparison with a single designed low noise amplifier, the efficiency of the low noise amplifier of the present invention is remarkably improved. The low noise amplifier is applicable to wireless communication standards and suitable for the frequency range devised in kinds of communication standards.